<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <p id="app">
        10:10:01
    </p>
    <script>
        function handleTime(value) {
            if (value < 10) {
                return "0" + value;
            }
            return "" + value;
        }
        function showTime() {
            var app = document.getElementById("app")
            var endTime = "2020-12-04 16:50:00";
    
            endTime = new Date(endTime).getTime();
      
            var nowTime = new Date().getTime();
   
            var gapTime = endTime - nowTime;

            var seconds = Math.floor(gapTime / 1000);
            var hours = handleTime(Math.floor(seconds / 3600));
            seconds = seconds % 3600;

            var minutes = handleTime(Math.floor(seconds / 60));
            seconds = handleTime(seconds % 60);
            var sum = hours + ":" + minutes + ":" + seconds;
            app.innerHTML = sum;
        }
        showTime();
        setInterval(function(){
            showTime();
        },1000)
    </script>
</body>

</html>